How the Penis Lost Its Spikes
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By Zoë Corbyn
Sex would be a very different proposition for humans if -- like some animals including chimpanzees, macaques and mice -- men had penises studded with small, hard spines.
Now researchers at Stanford University in California have found a molecular mechanism for how the human penis could have evolved to be so distinctly spine-free. They have pinpointed it as the loss of a particular chunk of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.
"It is a small but fascinating part of a bigger picture about the evolution of human-specific traits," said Gill Bejerano, a developmental biologist at Stanford who led the work along with colleague David Kingsley. "We add a molecular perspective to a discussion that has been going on for several decades at least."
Published in Nature today, the research also suggests a molecular mechanism for how we evolved bigger brains than chimpanzees and lost the small sensory whiskers that the apes -- who are amongst our closest relatives and with whom it has been estimated we share 96% of our DNA -- have on their face.
Monogamous strategy
It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known.
The researchers did not set out to study penile spines. Rather, they were looking for chunks of DNA that had been lost from the human genome but not the chimp genome, so they could then try to pinpoint what those chunks did.
The approach differs from that in most studies, explain Bejerano and Kingsley, in looking at what has been deleted from the human genome rather than what is present. "In the case of our study, had you started from the human genome, there would be nothing there to see," says Bejerano.
They first systematically identified 510 DNA sequences missing in humans and present in chimps, finding that those sequences were almost exclusively from the non-coding regions of the genome, between genes. They then homed in on two sequences whose absence in humans they thought might be interesting -- one from near the androgen receptor (AR) gene and one from near a gene involved in tumour suppression (GADD45G).
Inserting the chimpanzee sequences into mouse embryos revealed that the former sequence produced both the hard penile spines and sensory whiskers present in some animals. The latter sequence acted as a kind of brake on the growth of specific brain regions -- with the removal of its function appearing to have paved the way for the evolution of the larger human brain.
"The goal of the project was to find molecular lesions [losses] that underlie human evolutionary traits, with the examples illustrating different aspects of the principle," says Kingsley.
"Until we looked at where the DNA was expressed, we had no idea which switch -- if any -- it would actually control," adds Bejerano.
Other molecular biologists praised the work for its clever approach and said it would open up new avenues of inquiry, particularly for those working on the evolution of the human brain.
"As so often with very good ideas, it seems almost obvious in hindsight," said Svante Pääbo, who directs the genetics department of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and was part of the team that recently sequenced the Neanderthal genome. "Since two of the almost 500 deleted sequences they identified turn out to be interesting, I am sure that several other ones on their list will turn out to be interesting too," he added. The researchers are continuing to analyse the remaining 508 DNA sequences."It is detective work and a great reminder that, in the course of evolution, information is both gained and lost," said Sean Carroll, an expert in animal genetics and evolution at the University of Wisconsin, Madison.
David Haussler, who studies the molecular evolution of the human genome at the University of California, Santa Cruz, added that our ancestors' loss of penile spines is our gain today."Couples everywhere can be thankful that this particular piece of DNA was ditched," he says.
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24 Comments
Add CommentI've come across at least one published study that showed the human foreskin to be an extremely efficient plunger for the collection and subsequent removal of a competitor's sperm from the vaginal canal. This in light of the above findings suggests that the foreskin is what ultimately made human intelligence possible.
Reply | Report Abuse | Link to thisSomething obviously to do with the invention of the "French Tickler" condom. It was in our genes all along.
Reply | Report Abuse | Link to thisAny idea how we can restore the spikes?
Reply | Report Abuse | Link to thisWe urgently need population control.
Very Funny, Earthling.
Reply | Report Abuse | Link to thisI'd think that natural selection in all it's common sense would have encouraged this particular evolution . . . namely that human females would tend to be more accepting of the advances of the fellows with less spikes, and also less likely to club them in the head with the nearest rock. Thus of course leading to both the survival and to the birth of the less spiky males.
The limitation in intelligence is linked to the gene sequence associated with the sensory whiskers. The penile spines are thought to have given a reproductive advantage by removing competing sperm which will not promote intelligence unless its because the smart guy figured out that he needed to be last.
Reply | Report Abuse | Link to thisWho cares about penile spikes? This is the infinitely less interesting finding. I haven't looked at the paper yet, but if GADD45G does play a role in suppressing the growth of cells in forebrain structures, then this finding suggests a potentially key step in the evolution of human intelligence.
Reply | Report Abuse | Link to thisBut hey, who cares about brains when we can talk about sex, right?
Bett,
Reply | Report Abuse | Link to thisSounds rational. So how come spikes evolved in the first place?
Maybe the researchers should look in the chimp gene to find out why female chimps accept spiky peni and female human ancestors did not. Look for a loss of genes that meant that human female ancestors no longer found spikes stimulating, just a pain.
These researchers are so close. The evidence is all around them but they are looking with the usual blinkers imposed on them by the assumptions of their disciplines, based on reverential belief in Natural Selection, in turn based on Darwin's decision to reject the effects of external conditions. He decided that change resulted from variations that gave individuals superior internal qualities that favoured them in struggle for scarce resources. It follows that change results directly from emergence of variations and is continuous and incremental. The more variations the more change, so change must be faster in species with large populations which would have more variations.
Genetic evidence used in this report shows that those assumptions were wrong. The reverse is true. Change has resulted not from emergence of variations, but from elimination of some of those variations. More variations increase the spread of the bell curve around the mean, a sign of a LACK of evolutionary change.
What shifts the mean is another mechanism that eliminates variations on one side of the mean, survival pressure from a change in the conditions to which that species was adapted that eliminates individuals unable to survive the change.
Evolution results from the very factor that Darwin excluded. Many biologists agree, but continue to apply rules and assumptions based on rejection of external conditions.
They say that evolution can have no direction or purpose because their assumptions put attention on the individual, who cannot have purpose. It is external conditions that create direction (adaptation) and purpose (survival). External pressures find individuals whose behaviours are adapted to the new conditions by the very simple mechanism of eliminating individuals unable to survive. It is their elimination that evolves the species.
That is why chimps have had far less evolution and have far more variations in their genome. That is why, if you want to know why a male characteristic changed, look outside the male to what changed among females.
What will it take to convince biologists to remove the blinkers of incorrect assumptions so they can produce explanations of natural phenomona that make sense?
Davy@2: Can you provide the source of that study? The protocol may prove revealing and could merit further research.
Reply | Report Abuse | Link to thisOK, I apologize because this is totally unrelated, but I am really sick of all the people spamming ads into these comments... why can't SciAm figure out how to prevent the spam...?
Reply | Report Abuse | Link to thisFor example, when you submit comments, how about requiring one of those "type the letters you see in the box below" prompts...? Please SciAm, get this fixed!
tharriss,
Reply | Report Abuse | Link to thisThere is a simple method, just click Report Abuse. It disappears within a day, so we are the editors.
I think we might also have lost the "os penis" a small bone on the pelvis which requires we are bombarded with "erectile dysfunction" advertisements night and day. I object to them in "prime time" they make a chimps life look like fun! Besides Alice is a fine old-fashion name. Why should a primate's bone be so envied?
Reply | Report Abuse | Link to thisWow! The debate. How many scientist have on their mind. Premature ejaculation? A with studs? A without studs? A with speed bumbs? A without speed bumbs? How many humps/sec does it take for the average man to ejaculate? Do you dribble before or after you shoot? How many scientists does it take to study the human ?
Reply | Report Abuse | Link to thisIf smooth equal bigger brain and spiked equal small brain, does the smooth male have bigger feet and a spiked male have smaller feet. When did scientist find out that early man had a spiked ? And when did man lose his spikes? Spikes or no spikes. That is the question.
Krist,
Reply | Report Abuse | Link to thisI do not think that whatever happened to ancestral peni had much to do with the level of intelligence.
Then again, some of the comments that appear in Sci Am blogs might come from that region, rather than from between the ears.
@Tharriss Why swing all the way in the other direction? Why can not one be true and the other also? There is seemingly enough evidence for Darwin's mechanism. It creates the variation needed in the first place, whether through gene exchange via viruses, mutations in place, or other mechanisms. As you stated, environmental conditions pressure the selection toward the part of the more favored survivors. Both would be mechanisms of evolution. In fact, evolution is more reasonably explained by the existence of both working together.
Reply | Report Abuse | Link to thisYou may also want to examine if one can exist without the other. Darwinian individual modifications can grow inside a population without external pressures and may eventually lead to speciation. Yet, external pressures without Darwinian modifications will lead to extinction since no individuals may be adapt enough, the pool may be too small.
My apologies, this was directed at hoamingin.
Reply | Report Abuse | Link to thisNo thanks. Those "type the letters" are virtually unreadable and a big pain in the butt.
Reply | Report Abuse | Link to thisTrabant70,
Reply | Report Abuse | Link to thisI did not say that variations stop happening, just that Darwin assumed that change resulted directly from the emergence of favourable variations, leading him into a series of assumptions that genetics has found to be the opposite of what actually happens.
Variations do not lead directly to change. They accumulate as diversity, expanding the possibilities for future change, but what creates change is a separate mechanism.
That separate mechanism is survival pressure from change in external conditions that works on the accumulated diversity, eliminating individuals unable to survive the changed conditions, in the process eliminating part of that diversity.
That is what this study showed. Chimpanzees continued living in forests and had no major external pressures, so they remained largely unchanged and accumulated genetic diversity. Human ancestors led a vulnerable lifestyle on the ground and were forced into a change of lifestyle, hunter gatherer, and standing on two feet, freeing hands to use external objects. The greater range and flexibility of behaviours required evolved a larger brain by eliminating individuals who lacked those abilities.
Darwin knew that there could be no change in a species unless some individuals were removed. Having rejected the effect of external conditions, he attributed their extermination (his term) to the actions of favoured individuals.
I say that their elimination is THE mechanism that evolves the species towards individuals able to survive, in the process evolving the species more closely to those conditions. Darwin never explained why their extermination was essential for change, but he still claimed that change came from improved internal qualities that favoured individuals.
In both approaches the elimination of unsuccessful individuals is essential. My explanation attributes their demise to pressures from external conditions, Darwin's explanation attributes it to the actions of improved individuals. My explanation fits the genetic evidence, Darwin's explanation is contradicted by that evidence.
Geneticists continue to attribute evolution to Natural Selection, all of the assumptions of which their own science contradicts.
Go figure that one out.
Amazing. Since we are looking at the difference between apes and men then we are talking about relatively recent changes (say a few million years ago rather than tens or hundreds of million).
Reply | Report Abuse | Link to thisNow imagining the cause of these DNA eliminations to be random mutation, I still have difficulty imagining the massive change between us and the apes (and I seriously am more moved by brain growth and functionality than other organs).
I remember reading a case where non-sugar-digesting organisms which could mutate to sugar-digesting ones were placed in a sugar-rich environment, only to be observed that the rate of mutation itself changed drastically (I think it was in the order of 1000s or ten thousand times if memory serves me right). It was obvious that the mutation itself was not purely random but affected by external conditions. How? Who knows?
The book was about quantum physics and it "guessed" that this phenomenon "could be" related to quantum entanglement of Hydrogen atoms inside and outside the organisms. "Somehow" allowing exchange of information on what is available outside, though the organism has no cognition of what sugar is and how it could possibly be a source of food.
But the above finding amazes me in the same way. It seems to me it is the case of the sugar-digesting organisms all over again. In other words some outside circumstance affecting the process of DNA elimination to speed up the brain growth. Not just random mutations (aided by natural selection). Thus the speedy development of the human brain.
These are what I understand about this article......
Reply | Report Abuse | Link to this1. We lost the spine in the penis to encourage monogamous sex partnership.
2. This means the first sex partner has the greatest chance of fertilizing. This was not possible before when the spine was there to remove the semen of prior male sex partner. Also, this means the dominant male has to keep all other males from mating with his female partner or harem. Now, the male just have to be the first.
3. The DNA that has created the spine was ditched by our evolution to allow a greater growth to our human brain.
As I previously understand, other male animals has to be dominant to ensure the propagation of his DNA.
These are what I understand about this article......
Reply | Report Abuse | Link to this1. We lost the spine in the penis to encourage monogamous sex partnership.
2. This means the first sex partner has the greatest chance of fertilizing. This was not possible before when the spine was there to remove the semen of prior male sex partner. Also, this means the dominant male has to keep all other males from mating with his female partner or harem. Now, the male just have to be the first.
3. The DNA that has created the spine was ditched by our evolution to allow a greater growth to our human brain.
As I previously understand, other male animals has to be dominant to ensure the propagation of his DNA.
KamranBehzad,
Reply | Report Abuse | Link to thisThe human line separated from the line that produced gorillas and chimps about 7mya, though that might change if someone digs up bones that show a different date. But it was a long time ago, plenty of time to work through the stages of bipedal apes, hominids, early humans etc.
The big increase in brain size was over 2mya, facilitated by a mutation of the MYH gene that had locked hominids into heavy ape jaws, strong chewing muscles and thick skull to anchor them. The first humans quite quickly evolved thinner skulls that grew in size, with a large brain.
Using your analogy of sugar and non sugar eating organisms, what evolves must already exist as a variant in a population. Change the conditions and those individuals unable to survive will be eliminated, evolving the species. The rate of change you mention might relate to composition of the population, which can change rapidly, rather than change in the genome.
The more extreme the change in conditions, the greater the survival pressure on the species is likely to be, the faster the change and the more tightly adapated to the changed conditions the species will become.
Bacteria do not evolve resistance to antibiotics, antibiotics evolve resistance in bacteria by killing all variants of the bacteria except those able to survive the effects of the antbiotic.
Apply that to humans and hominids must have been under a lot of pressure to have a wider range and flexibility of behaviours. Geneticists tell us that the lack of diversity in the human genome shows that our ancestors may have come close to extinction. That sounds like strong survival pressure. It just happened that there were some individuals with larger brains and breeding among individuals with similar characteristics in a small population will exaggerated those characteristics.
Change is not dependent on the rate of mutation. Chimps have higher rates of mutation and far greater diversity than humans. What they have not had is external pressure that eliminated some of that diversity, reshaping the species. They stayed in the forests and stayed as chimps.
Raseclamid,
Reply | Report Abuse | Link to thisYour comment made me look hard at the article. I also looked at the original article in Nature.
http://www.nature.com/nature/journal/v471/n7337/full/nature09774.html
A major point is that some of the things that make humans unique have not been additions to human DNA, but deletions of bits from DNA, which may have removed some inhibition or control on various processes.
I think you are making too many direct cause and effect relationships among the data. The article does not say definitively that we lost spines to encourage monogamy, it repeats earlier speculation about that, which may, or may not be so. As I suggested earlier, the reason males lost spikes may have been another change in females that made spikes a pain, so spiky males did not get many chances to propagate.
The article mentions two deletions, one that affects penile spines and sensory facial whiskers, and another that affects brain development, so two separate things.
Hi Hoamingin! Not sure I agree with you here. Darwin did go on to write a lot about SEXUAL selection, after his original NATURAL selection book, which seems to be quite relevant here. Also he didn't know ANYTHING about genetics (and said so in so many words, as one of his great problems with his own theory!), so he had NOTHING to say about addition/deletion of genetic information. But he DID give quite a lot of examples of the loss as well as additions, of PHYSICAL traits,such as eyes.
Reply | Report Abuse | Link to thisI think he gets a bad press sometimes! When I read his two main books, I am really struck with just how prescient he was (as well as how good a 'popular' writer), considering his own very honest acknowledgement of the lack of some critical evidence and knowledge at the time (genetics being a prime example, as he apparently did not come across Mendel's work)).
But he was obviously not right on everything, and people like Richard Dawkins (surprisingly!) make clear those inaccuracies. It's just impressive how logical most of his work was, and how well he presented his arguments and evidence.
OK, so I enjoy reading his books!!
Bigperk,
Reply | Report Abuse | Link to thisDarwin's great insights included recognising that individuals differ to some degree and a mechanism had, over long periods of time, operated on those differences to evolve all existing species from common origins.
Thomas Huxley recognised those propositions as logical and scientific. He wrote to Darwin the day before publication pledging his support, but also put in writing his reservation about Darwin's explanation, questioning why he gave such little importance to external conditions. Darwin replied "if, as I must think, external conditions produce little direct effect, what the devil determines each particular variation?” Why MUST he think that? Not for scientific reasons, he was conforming to the beliefs of the culture he grew up in, seen in the writings of Malthus, Spencer and Galton.
Reread ch. 3 of Origin and try to follow his argument that species do not adapt to conditions because of the conditions, but because species are favoured in a struggle for existence in those conditions, without any effect from the conditions. There are many such logical gaps throughout Origin.
In 1888 Huxley wrote about Origin “It is one of the hardest books to understand thoroughly that I know of.” Huxley recognised there was something wrong with Darwin's logic, but did not put his finger on it. If you reread Origin with the understanding that Darwin specifically excluded external conditions from Natural Selection, you will spot the gaps in logic.
Darwin's decision forced him and generations of scientists, when explaining phenomena, to focus on successful individuals to the exclusion of the context in which it happens. Sexual Selection was an example of the failure of his explanation, so he made up another one that attributes male appearance and behaviour to males competing for mating, ignoring the role of irrational female preferences for non-adaptive features in their mating partners.
Instead of devoting time to exploring explanations that better explain the data, Dawkins drove Darwin's errors into genetics. In response to criticisms, Dawkins wrote "the idea of animals behaving as if calculating odds without really doing so is fundamental to an understanding of the whole of sociobiology". This may be the only science in which pretence is essential to understanding it.
This pretence and the focus on individuals to the exclusion of context flow from Darwin's decision to exclude external conditions and attribute change to internal qualities of individuals. See if your understanding of Origin change when you reread it.